1. Field of the Invention
The present invention relates to an arteriostenosis diagnosing apparatus which diagnoses arteriostenosis of a living subject based on an inferior and superior limb blood pressure index, and pulse wave velocity related information, obtained from the subject.
2. Related Art Statement
Inferior and superior limb blood pressure index is known as an index useful to diagnose arteriostenosis or arterial occlusion resulting from atherosclerosis. This index is expressed as the ratio of superior limb blood pressure to inferior limb blood pressure or the ratio of inferior limb blood pressure to superior limb blood pressure. Generally, systolic blood pressure values are measured to determine this index. In addition, generally, an ankle is used as the inferior limb and a brachium is used as the superior limb, i.e., ankle and brachium blood pressure index ABI is measured as the inferior and superior limb blood pressure index. When a certain portion of an artery has stenosis, a blood pressure in a downstream portion of the artery that is located on a downstream side of the stenotic portion in a direction of flow of blood in the artery is lower than that of an upstream portion of the artery, so that an abnormal inferior and superior limb blood pressure index is obtained and accordingly arteriostenosis can be diagnosed. In many cases, arteriostenosis occurs to an inferior limb of a living being.
Since inferior and superior limb blood pressure index is a simple index, i.e., the ratio of one of inferior and superior limb blood pressure values to the other, it is needed to measure accurate inferior and superior limb blood pressure values so as to obtain the index with reliability. By the way, calcification is a different arteriosclerosis than atherosclerosis. When calcification of a certain portion of an artery progresses, a blood pressure in the calcified portion is higher than that in other portions of the artery. If the calcification further progresses, the blood pressure of the calcified portion becomes still higher, because the calcified portion cannot completely be occluded. Therefore, if calcification of artery progresses in an inferior limb, a normal inferior and superior limb blood pressure index may be obtained even if the inferior limb may have arteriostenosis.
Hence, when a normal inferior and superior limb blood pressure index is obtained, it is needed to judge whether the normal index means that the inferior limb does not have arteriostenosis, or that the inferior limb has not only arteriostenosis but also calcification. To this end, there has been proposed an apparatus which obtains, in addition to inferior and superior limb blood pressure index, pulse wave velocity related information that is related to a velocity at which a pulse wave propagates between two body portions of a living being. This apparatus is disclosed by, e.g., Patent Document 1 (Japanese Patent No. 3,140,007). Since pulse wave velocity related information is useful to evaluate the degree of calcification of artery, whether arteriostenosis is present or not can be judged more reliably based on the pulse wave velocity related information in addition to the inferior and superior limb blood pressure index.
Thus, the purpose of measurement of the pulse wave velocity related information is to determine the degree of calcification of the inferior-limb artery from which the inferior limb blood pressure value used to calculate the inferior and superior limb blood pressure index is measured. Therefore, recently, it is proposed to obtain pulse wave velocity related information from an interval whose one end is defined by a measurement portion (e.g., an ankle) of an inferior limb where an inferior limb blood pressure value is measured, and which includes an upstream portion of the inferior limb that is located on an upstream side of the measurement portion. This technique is proposed by, e.g., Patent Document 2 (Japanese Patent Publication No. 2002-272688).
However, as the degree of arteriostenosis increases, the pulse wave velocity related information is influenced more and more by the stenosis, and the direction of influence of the stenosis to the information is opposite to that of influence of the calcification. For example, as arteriostenosis progresses, pulse wave velocity decreases; on the other hand, as calcification progresses, the velocity increases. Therefore, in the case where pulse wave velocity related information is obtained from the interval including the upstream portion of the inferior limb, located upstream of the measurement portion where the inferior limb blood pressure is measured, as taught by Patent Document 2, normal pulse wave velocity related information may be obtained depending on respective degrees of arteriostenosis and calcification. Thus, there are some cases where, though arteriostenosis is present, neither pulse wave velocity related information nor inferior and superior limb blood pressure index show abnormal values and accordingly the stenosis cannot be found.